High-stability frequency reference based on self-locked alkali-vapor laser

Abstract

Embodiments of the invention are directed to an atomic-based frequency reference that includes an architecture that eliminates the need for local oscillator components and instead is implemented with a “photonic local oscillator,” that provides for a relatively low power consumption, compact, atomic clock/frequency reference. Such a system is based on a laser cavity which contains an alkali-vapor-cell within the laser cavity. The laser cavity is designed to oscillate in two or more optical wavelengths, with the frequency difference of these lasing wavelengths equal to the atomic hyperfine resonance frequency. Embodiments of the system can include relatively few elements, namely: a) an optical gain element with an emission band centered at the atomic absorption band; b) collimating optics; c) an atomic vapor cell; and d) a cavity end mirror. Optionally, specific polarization optics may be used depending on the specifics of the optical pumping.

1 . An external cavity laser system comprising:
 a cavity having a plurality of cavity modes;   an alkali-vapor cell disposed within the cavity;   a gain medium disposed within the cavity, and adjacent the alkali-vapor cell; and   an output coupler disposed within the cavity, and adjacent the alkali-vapor cell;   wherein the gain medium, the alkali-vapor cell, and the output coupler are arranged and configured such that two or more of the cavity modes are phase locked.   
   
   
       2 . The system of  claim 1 , wherein the cavity mode spacing is matched to the hyperfine frequency of the alkali-vapor cell. 
   
   
       3 . The system of  claim 1 , wherein the phase locked cavity modes are line-narrowed by nonlinear gain discrimination. 
   
   
       4 . The system of  claim 1 , further comprising:
 a high speed photodetector to convert the phase locked cavity modes to a stable radio frequency (RF) frequency.   
   
   
       5 . The system of  claim 4 , wherein the stable RF frequency is line narrowed, producing a linewidth narrower than natural alkali-atom linewidth, 
   
   
       6 . The system of  claim 1 , further comprising:
 a high speed photodetector to convert the phase locked cavity modes to a stable clock output.   
   
   
       7 . The system of  claim 1 , wherein the output coupler comprises a grating reflector, the grating reflector being shaped to narrow the optical cavity bandwidth. 
   
   
       8 . The system of  claim 4 , further comprising:
 a crystal oscillator, wherein the output from the photodetector is used to as a master reference by which the crystal oscillator is disciplined.